1. Field of the Invention
The invention relates to a method for providing uniform and warp-free folds in light gauge polymer sheet materials and finds particular application in the manufacture of jackets for flexible magnetic disks.
2. Description of the Prior Art
Flexible magnetic disks of various configurations have been commercially available since the early 1970's. In order that information recorded upon a particular flexible magnetic disk be readable upon a variety of flexible magnetic disk drives, certains standards for disk uniformity and dimensions have been developed. One such standard configuration is the American National Standards Institute (ANSI) X3B8/77-118 (June 27, 1977) specification.
Flexible magnetic disks must operate with certain mechanical and electrical characteristics so that the integrity of the data is maintained. For example, there is a torque specification which requires that the resistance to rotate the flexible magnetic disk within the flexible magnetic disk jacket be within predetermined values. Rotating torques in excess of the specification can create data reliability problems. In addition, the jacket which encloses and protects the flexible magnetic disk must be flat within specified tolerance limits. Jackets which are not flat within the allowable extremes tend to cause excessive torque to rotate the flexible magnetic disk within the jacket and to cause data reliability problems due to head-to-disk contact instability and/or separation. The material from which the flexible magnetic disk jackets have been universally constructed is polyvinyl chloride (PVC).
PVC will meet the technical criteria as a material from which to construct the flexible magnetic disk jacket in accordance with ANSI X3B8/77-118. However, PVC has a deformation or "softening" temperature of approximately 125.degree. F. For applications in which the flexible magnetic disk will be exposed to temperatures in excess of 125.degree. F., the PVC jacket tends to warp and become generally unusable destroying any possibility of recapturing the data stored upon the flexible magnetic disk. For example, military applications often are such that extreme temperatures must be sustained, and PVC has proven unacceptable. It has thus been known for some time that other materials that could withstand higher temperatures without deforming would be desirable to extend the useful operating range of flexible magnetic disks.
A material which is readily available and highly desirable for use as a flexible magnetic disk jacket is a polycarbonate known commonly as Lexan. Lexan polycarbonate has a deformation "softening" temperature of 321.degree. F. It is a much "tougher" material than PVC. However, manufacturers of flexible magnetic disks have been unable to successfully produce a Lexan flexible magnetic disk jacket primarily due to the difficulties in forming light gauge Lexan sheet so that the folded structure of the jacket meets the flatness criteria of ANSI X3B8/77-118. The readily available literature supplied by the General Electric Company, manufacturer of Lexan light gauge polycarbonate sheet material, is wholly inadequate to produce a suitably formed Lexan flexible magnetic disk jacket. Preheating the Lexan sheet as suggested by General Electric and subsequently forming and/or folding such sheets into the desired jacket structure invariably results in a warped jacket that is out of ANSI tolerances. Moreover, the available technical literature for forming and/or folding polycarbonate light sheet material does not indicate any practical heating and/or bending fixturing that would aide in achieving a structure useful in the flexible magnetic disk application.
The prior art does describe techniques for producing stress free structures constructed from polymer materials, but such disclosures are generally not applicable to the forming of light gauge polymer sheet materials, especially Lexan polycarbonate. For example, U.S. Pat. No. 3,555,135 issued to Paul describes a method for molding substantially rectilinear-shaped structures from thermoplastic polymers with the improvement being stress relief at the corners of the structures so formed by the localized application of heat. According to the teachings of Paul, heat is applied in a generally uncritical manner to the corner of structures after said structures have been injection molded. The amount of heat applied and the time for application of such heat is sufficient so that the temperature of the polymer is raised to a temperature not exceeding 1.degree. less than the crystalline freeze point of the material from which the structure is molded but not lower than 50.degree. below said crystalline freeze point. While the concept of thermally induced stress relief at the corners of polymer structures is clearly disclosed by Paul, the invention is directed to injection molded parts and not to structures formed from commonly available polymer sheet material. Moreover, Paul does not disclose any details relating to the uniformity with which the heat must be applied to achieve the desired stress relief or the details of any heating or bending fixturing.
U.S. Pat. No. 3,200,182 issued to Hechelhammer et al describes a method for stress relieving a shaped polycarbonate structure. As was true in Paul, the concept of stress relief of a polymer through the use of heat is old in the art. Hechelhammer et al indicates that polycarbonate can be stress relieved if it is subjected for a long period of time to a temperature of from approximately 248.degree. F. to 293.degree. F. (i.e., approximately 120.degree. C. to 145.degree. C.). Hechelhammer et al indicates that approximately one hour of such heat treatment is required per millimeter of wall thickness. As this is practicably unacceptable, Hechelhammer et al teaches the exposure of the polycarbonate article to a temperature of from approximately 572.degree. F. to 2192.degree. F. (i.e., approximately 300.degree. C. to 1200.degree. C.). The exposure to these relatively extremely high temperatures is for a rather short duration. Hechelhammer et al is directed to the rapid stress release of injection molded or extruded polycarbonate structures and contains no teachings which relate to the bending or folding of light gauge polycarbonate sheet material. Moreover, Hechelhammer et al contains no teachings which relate to achieving extreme dimensional stability for a formed or folded structure. Moreover, it has been found in practice that use of temperatures in excess of 375.degree. F. (i.e., 191.degree. C.) cause bubbling of polycarbonate sheets of the Lexan type.
In the specific area of Lexan polycarbonate flexible disk jackets, Memorex Corporation manufactures a model FD-65 flexible magnetic disk with a Lexan jacket. However, this flexible magnetic disk jacket is not folded, but rather the jacket is made from a composite of a plurality of Lexan polycarbonate flat layers.